1. Field of the Invention
This invention relates to illumination devices and, more particularly, to illumination devices comprising a plurality of light emitting diodes (LEDs) and to methods for calibrating and compensating individual LEDs in an illumination device, so as to maintain a desired luminous flux and/or a desired color point of the device over variations in temperature and process while avoiding undesirable visual artifacts, such as brightness banding and flicker.
2. Description of the Relevant Art
The following descriptions and examples are provided as background only and are intended to reveal information that is believed to be of possible relevance to the present invention. No admission is necessarily intended, or should be construed, that any of the following information constitutes prior art impacting the patentable character of the subjected mater claimed herein.
Lamps and displays using LEDs (light emitting diodes) for illumination are becoming increasingly popular in many different markets. LEDs provide a number of advantages over traditional light sources, such as incandescent and fluorescent light bulbs, including low power consumption, long lifetime, no hazardous materials, and additional specific advantages for different applications. When used for general illumination, LEDs provide the opportunity to adjust the color (e.g., from white, to blue, to green, etc.) or the color temperature (e.g., from “warm white” to “cool white”) to produce different lighting effects. In addition, LEDs are rapidly replacing the Cold Cathode Fluorescent Lamps (CCFL) conventionally used in many display applications (such as LCD backlights), due to the smaller form factor and wider color gamut provided by LEDs. Organic LEDs (OLEDs), which use arrays of multi-colored organic LEDs to produce light for each display pixel, are also becoming popular for many types of display devices.
Although LEDs have many advantages over conventional light sources, a disadvantage of LEDs is that their output characteristics tend to vary over temperature, process and time. For example, it is generally well known that the luminous flux, or the perceived power of light emitted by an LED, is directly proportional to the drive current supplied thereto. In many cases, the luminous flux of an LED is controlled by increasing/decreasing the drive current supplied to the LED to correspondingly increase/decrease the luminous flux. However, the luminous flux generated by an LED for a given drive current does not remain constant over temperature and time, and gradually decreases with increasing temperature and as the LED ages over time. Furthermore, the luminous flux tends to vary from batch-to-batch, and even from one LED to another in the same batch, due to process variations.
LED manufacturers try to compensate for process variations by sorting or binning the LEDs based on factory measured characteristics, such as chromacity (or color), luminous flux and forward voltage. However, binning alone cannot compensate for changes in LED output characteristics due to aging and temperature fluctuations during use of the LED device. In order to maintain a constant (or desired) luminous flux, it is usually necessary to adjust the drive current supplied to the LED to account for temperature variations and aging effects.
Many LED manufacturers have recognized a need for temperature compensation, and there are several different ways in which temperature compensation is currently implemented in today's LED devices. However, most of these implementations follow the same, or roughly the same, temperature compensation method. For example, most temperature compensation methods begin by measuring the temperature of an LED or a string of LEDs. In some cases, one or more temperature sensors may be arranged near the LEDs to measure the ambient temperature surrounding the LEDs, or heat sinks may be coupled to the backside of the LEDs to measure the heat generated thereby. While heat sinks are generally needed for thermal dissipation, adding temperature sensors to the chip unnecessarily increases the cost of the LED device and consumes valuable chip real estate. More importantly, the temperature sensors and heat sinks added to the chip often cannot provide an accurate temperature measurement for all LEDs included with the LED device.
For example, many LED devices combine different colors of LEDs within the same package to produce a multi-colored LED device. An example of a multi-colored LED device is one in which two or more different colors of LEDs are combined to produce white or near-white light. There are many different types of white light lamps on the market, some of which combine red, green and blue (RGB) LEDs, red, green, blue and yellow (RGBY) LEDs, white and red (WR) LEDs, RGBW LEDs, etc. By combining different colors of LEDs within the same package, and driving the differently colored LEDs with different drive currents, these lamps may be configured to generate white light or near-white light within a wide gamut of color points or color temperatures ranging from “warm white” (e.g., roughly 2600K-3700K), to “neutral white” (e.g., 3700K-5000K) to “cool white” (e.g., 5000K-8300K).
However, the drive currents supplied to the differently colored LEDs in a multi-colored LED device can vary significantly from one another, depending on the desired color temperature. For instance, when an RGB lamp is configured for producing 2700K warm white light, the drive current supplied to the blue LEDs can be less than 10% of the drive current supplied to the red LEDs. Since an LED driven with a significantly higher drive current necessarily produces more thermal power, the junction temperature (i.e., the temperature of the active p-n region) of the red LEDs, in this instance, can be significantly greater than the junction temperatures of the blue and green LEDs. In some cases, the junction temperature of differently colored LEDs within the same package can differ by 5° C. or more, even with the same heat sink temperature. Therefore, it is usually more desirable to measure or estimate the LED junction temperatures, as opposed to the ambient or heat sink temperatures, and adjust the individual drive currents accordingly to maintain a precise color point produced by a multi-colored LED device.
It is generally well known that the forward voltage of an LED changes linearly with junction temperature when a fixed forward-biased drive current is supplied to the LED. FIG. 14 demonstrates the linear relationship between forward voltage and junction temperature with the forward voltages normalized to ‘1’ at 25° C. (roughly room temperature). As shown in FIG. 14, the forward voltage developed across the LED junction decreases linearly as the junction temperature increases (and vice versa). As a consequence, LED forward voltages measured at a fixed drive current can be used to provide a fairly precise estimate of junction temperature for a particular LED.
However, most manufacturers of conventional LED devices fail to account for the fact that the magnitude and slope of the line correlating forward voltage to junction temperature (shown, e.g., in FIG. 14) can vary significantly between LED manufacturers, LED part numbers and even individual LEDs arranged side by side on the same chip. To illustrate this point, the dotted lines shown in FIG. 15 show a possible range of forward voltage versus temperature characteristics that may be seen from a particular manufacturer and part number, while the solid line indicates the forward voltage versus temperature line generated by an individual LED from that manufacturer and part number. As shown in FIG. 15, the magnitude of the forward voltage can vary significantly between individual LEDs at any given temperature. In addition, FIG. 15 shows that the slope of the line relating forward voltage to temperature can vary between individual LEDs. While the differences in slope are typically small, they can represent a few degrees C. measurement error over the operating temperature range of an LED. These measurement errors result in inaccurate temperature compensation if steps are not taken to account for these variations when calibrating conventional LED devices.
In addition to variations in forward voltage, most manufacturers fail to account for the non-linear relationship between luminous flux and junction temperature for certain colors of LEDs, and the non-linear relationship between luminous flux and drive current for all colors of LEDs. Without accounting for such non-linear behavior, conventional multi-color LED devices cannot be used to provide accurate temperature compensation for all LEDs included within the multi-color LED device.
For example, FIGS. 16 and 17 illustrate the relative change in luminous flux over junction temperature produced by differently colored LEDs supplied with fixed drive currents. As shown in FIG. 16, the luminous flux produced by green, blue and white LEDs changes relatively little and linearly with changes in junction temperature. However, FIG. 17 shows that the luminous flux produced by red, red-orange and, especially, yellow (amber) LEDs changes significantly and sometimes dramatically over temperature, and that these changes are substantially non-linear. In order to provide accurate temperature compensation, the drive currents supplied to each color of LED must be individually calibrated and adjusted during use of the device. Conventional multi-color LED devices fail to provide calibration and compensation for each color of LED used in the device, and thus, fail to provide accurate temperature compensation in a multi-color LED device.
FIGS. 18 and 19 illustrate typical relationships between luminous flux and LED drive current for different colors of LEDs (e.g., red, red-orange, white, blue and green LEDs). As shown in FIGS. 18 and 19, the relationship between luminous flux and LED drive current is non-linear for all colors of LEDs, and this non-linear relationship is substantially more pronounced for certain colors of LEDs (e.g., green LEDs). Without accounting for such non-linear behavior, conventional LED devices cannot be used to provide accurate temperature compensation for all LEDs included within the LED device.
In addition to failing to account for non-linear behavior and differences in output characteristics between individual LEDs, conventional LED devices typically use pulse width modulation (PWM) dimming to control the overall luminance of the LED device. In PWM dimming, the duty cycle of the drive current (i.e., the ratio of time the drive current is “on”) is adjusted to control the overall luminance of the LED device. However, PWM dimming can be undesirable for a number of reasons. On a human level, pulse width modulation at certain frequencies has been shown to induce seizures and eye strain in some people. On a more technical level, PWM dimming causes issues for the power supply and the LEDs when switching large amounts of currents on and off. For example, in order to prevent the output voltage from varying too much, a larger output capacitor may need to be coupled across the power supply, which adds cost and consumes board space. However, this does not address the transients that occur in the drive currents supplied to the LEDs whenever the drive currents are turned on and off. In some cases, these transients can be visible in the form of flicker or color shift.
Another issue arises, not only when using PWM dimming, but whenever groups of LEDs are periodically turned on and off for any reason in an LED array. Whenever LEDs are periodically turned on and off, even at an imperceptibly high rate, an undesirable artifact called “brightness banding” occurs. This banding artifact is demonstrated in the photographs of FIGS. 20 and 21 as alternating bands of light and dark areas on a display screen backlit by an array of LEDs. The photograph shown in FIG. 20 was taken with a slow shutter speed to illustrate what the human eye sees, while the photograph shown in FIG. 21 was taken with a higher shutter speed to illustrate the bright and dark bands that develop across the display screen as a result of PWM dimming. This banding artifact also occurs when the light emitted by LEDs is modulated or turned on/off for other reasons, such as when modulating light output to communicate data optically in visible light communication (VLC) systems.
A need exists for improved illumination devices and methods for calibrating and compensating individual LEDs included within an illumination device, so as to maintain a desired luminous flux and/or color point of the device over variations in temperature and process. In order to overcome the disadvantages and inaccuracies associated with conventional methods, the calibration and compensation methods described herein take into account and adjust for variations in forward voltage magnitude and slope between individual LEDs, the non-linear relationship between luminous flux and junction temperature for certain colors of LEDs, and the non-linear relationship between luminous flux and drive current for all colors of LEDs. This enables the present invention to provide a more highly precise method of temperature compensation. Further, accurate temperature compensation is provided herein without producing undesirable visual artifacts, such as brightness banding, flicker and color shift.